Blood is a complex medium containing both cellular and non-cellular components. The principal protein of the cellular components is haemoglobin which transports oxygen. The analysis of patient's blood to determine the total amount of haemoglobin (tHb) and the different haemoglobin fractions is useful to a physician in evaluating the respiratory status of the patient. Examples of haemoglobin fractions are oxygenated haemoglobin (oxyhaemoglobin, O2Hb), oxidized haemoglobin (methaemoglobin or haemiglobin, MetHb), haemoglobin complexed with carbon monoxide (carboxyhaemoglobin, COHb), reduced haemoglobin (deoxyhaemoglobin, HHb), haemoglobin complexed with sulfur (sulfhemoglobin, SHb) and haemoglobin complexed with cyanide (cyanmethaemoglobin, CNMetHb). In general, O2Hb oxidizes over time, becoming MetHb. MetHb cannot bind oxygen, which is detrimental to the subject in which the MetHb has formed. For this, most organisms feature enzymes such as MetHb reductase, which convert MetHb back to O2Hb.
“CO-oximetry,” sometimes referred to as cooxymetry, generally refers to the process, often automated, in which a plurality of wavelengths is used to quantitate several haemoglobin fractions, in the same sample. The number of wavelengths required is equal to or greater than the number of haemoglobin fractions in the sample. The specific wavelengths employed depend, at least in part, on the haemoglobin fractions to be determined, the spectral response curves thereof and the quality of the filters or diffraction gratings used to isolate light having the selected specific wavelengths.
In order to ensure accurate results, a CO-oximeter requires a clearly defined quality control program. Such a program typically includes the analysis of samples having known concentrations of the various haemoglobin fractions. For these samples, in which the simultaneous presence of MetHb and O2Hb is required at known and reliable concentration levels, it is a problem that O2Hb can oxidize to MetHb. This oxidation renders neither the O2Hb concentration nor the MetHb concentration reliable or constant. In such samples, the use of MetHb reductase to regenerate O2Hb that has oxidized is not a solution to this problem, because it substantially depletes the desired concentration of MetHb as well. Access to good and reliable CO-oximetry control samples, which preferably feature both MetHb and O2Hb, is desirable.
Most CO-oximetry controls are based on dyes (see for example EP 132 399). Such controls are not attractive as their spectrophotometric characteristics are not the same as those of haemoglobin fractions in blood samples.
Other CO-oximetry controls are hemolysate based controls (see for example U.S. Pat. No. 4,485,174, US 2003/0068822 or US 2012/0104323). Using these controls it is not possible to have different constant and known levels of O2Hb and MetHb in the same sample.
A lyophilized bovine haemoglobin preparation containing a mixture of various fractions of O2Hb, COHb and MetHb had also been used as CO-oximetry controls. This preparation is assumed to be spectrophotometrically equivalent with a fresh bovine haemoglobin solution (Maas et al, Clinical Chemistry, 44:11, 2331-2339 (1998). However, this preparation needs to be reconstituted before use and lyophilization process and/or additives used for lyophilization may cause matrix effects.
Therefore there is still a need for improved CO-oximetry controls that do not have the drawbacks of the ones of the prior art. The CO-oximetry controls of the invention are liquid, ready-to-use, have different, custom, and reliable levels of O2Hb, COHb and MetHb and no or minimal matrix effect is expected when using the controls on different CO-oximetry analyzers.